Administration of cytotoxic chemotherapy is frequently limited by systemic toxicity. Accordingly, a number of strategies for the treatment of tumours in cancer patients have been developed with the aim of improving specificity of cytotoxicity effects such that tumour cells are selectively killed whilst normal, non-tumour, cells are undamaged.
Such strategies include enzyme prodrug therapy and stimulation of cancer cell-specific immune responses.
Enzyme prodrug therapy is a two-step approach: in the first step a drug activating enzyme is targeted and expressed in tumours; in the second step, a non-toxic prodrug which is a substrate of the exogenous drug activating enzyme is administered systemically. The non-toxic prodrug will be converted into the active anticancer drug at high concentration in the local environment of the tumour and thus tumour cells are killed while the systemic drug concentrations are minimised. A number of different enzyme prodrug therapy approaches are reviewed, for example, by Xu and McLeod, Clinical Cancer Research, November 2001, Vol. 7; 3314-3324.
One such approach is exemplified by P450/CYP2B6 expression for the activation of the pro-drug, cyclophosphamide. MetXia-P450 (Oxford Biomedica, Oxford, U.K.) is a novel replication deficient retroviral vector enabling the delivery and subsequent expression of the cytochrome-P450 2B6 gene (CYP2B6) for activation cyclophosphamide within cancer cells (reviewed in Kan, Expert Opin Biol Ther., 2: 857-868, 2002).
After oral or intra-venous administration, cyclophosphamide undergoes metabolism by cytochrome-P450 enzymes (primarily in the liver and to a lesser extent the lung and renal cortex) to 4-hydroxycyclophosphamide and aldophosphamide and then to phosphoramide mustard and acrolein (Colvin, Cancer Treat Rep., 65 Suppl 3: 89-95, 1981; Chang, Cancer Res., 53: 5629-5637, 1993). Phosphoramide mustard is an alkylating agent that induces DNA cross links and strand breaks. Most normal tissues are protected from the activation of cyclophosphamide by the detoxifying effects of aldehyde dehydrogenase (ALDH) and glutathione-S-transferase (GST) that convert aldophosphamide to the inactive carboxyphosphamide. ALDH is frequently absent from cancer cells but may be upregulated in tumours resistant to cyclophosphamide (Hilton, Cancer Res., 44: 5156-5160, 1984; Russo, Cancer Res., 48: 2963-2968, 1988; Russo, Prog Clin Biol Res., 290: 65-79, 1989; Chen, Biochem Pharmacol., 49: 1691-1701, 1995). Experiments in rats demonstrated that stable cell lines transfected with cytochrome P450 2B1 could be made sensitive to cyclophosphamide (Clarke, Cancer Res., 49: 2344-2350, 1989; Chen, Cancer Res., 55: 581-589, 1995). Studies with the human homologue, CYP2B6, confirmed this to be the most efficient P450 isoform for induction of cyclophosphamide mediated cytotoxicity (Chang, 1993; Code, Drug Metab Dispos., 25: 985-993, 1997; Jounaidi, Cancer Res., 58: 4391-4401, 1998). Direct delivery of cytochrome P450 enzymes to tumour cells should increase local activation of cyclophosphamide leading to greater cell kill and less normal tissue toxicity.
Other approaches for stimulating anticancer response have focussed on generating anti-tumour immune responses to human tumours through identifying specific antigens which single out the tumour cells from non-tumour cells such that an immune response is targetted to the unwanted cells. Methods employed include vaccination with a tumour-specific or tumour associated antigen to establish an anti-tumour response. Such methods are reviewed, for example, in Platsoucas et al. AntiCancer Research 23: 1969-1996 (2003).
These different approaches have in common the aim of generating a specific anti-tumour response through specific activation of an enzyme product in a tumour or through eliciting a specific, predetermined anti-tumour immune response to a specific tumour antigen.
However, the most common reason for failure of any anti-tumour therapy is the inability for the response to be maintained and to eliminate any secondary tumours which may develop. Such secondary tumours may differ from the primary tumour in the range of tumour antigens that are expressed or they may be more difficult to target with a construct for expressing a specific enzyme in an enzyme-prodrug approach.
Accordingly, there is a need for a method of treating human tumours that allows a more general response to be elicited against both the primary and secondary tumours.